1. Field of the Invention
The invention is in the field of heat-mirrors and more particularly in the field of heat-mirrors having high infrared reflectivity and high visible transmission.
2. Description of the Prior Art
Heat-mirrors that reflect radiation in the infrared spectrum while transmitting radiation in the visible spectrum have important applications as transparent thermal insulators for furnaces, windows in buildings, and solar-energy collection. In the field of solar energy, for example, it is desirable to collect sunlight efficiently and to convert it into heat energy. Traditional strategy for optimizing thermal energy collection with a flat-plate collector has been concentrated on the absorber wherein the sun's radiant energy is converted into heat. If the absorber is a black body, it converts all of the incident radiation into heat, but it also converts the heat to infrared radiation that is reradiated back into space. Usual strategy is to design a material that has high absorptivity for radiation from the sun, but that has low emissivity for infrared radiation. For solar energy farms to collect solar energy for thermal to electrical power conversion, this requires a material that has a low emissivity for infrared radiation from a body at about 800.degree. K. Moreover, the absorber material possessing these properties must be stable at such temperatures for long periods of time, must withstand thermal variations from cold winter nights to 800.degree. K during the day; and must also be cheap to manufacture and to maintain in the field. The probability of success with this strategy alone appears minimal.
An alternate strategy is to introduce a heat-mirror that is separated from the heat absorber and will therefore be much cooler. Heat-mirrors designed for this purpose have been fabricated from tin-doped indium oxide and antimony-doped tin oxide. See Groth, R. and Kauer, E., Philips Tech. Rev., 26, 105 (1965); Groth, R., Phys. Stat. Solid, 14, 69 (1966); Fraser, D. B. and Cook, H. D., J. Electrochem. Soc., 119, 1368 (1972); Vossen, J. L. and Poliniak, E. S., Thin Solid Films, 13, 281 (1972); Mehta, R. R. and Vogel, S. F., J. Electrochem. Soc., 119, 752 (1972); and Vossen, J. L., RCA Review, 32, 289 (1971). Although these materials are stable in air up to 400.degree.-500.degree. C. and can have visible transmissions as high as 80-90% on Pyrex glass, their infrared reflectivities at around 10 micrometers (for room temperature radiation) are only between about 80-90%, which are much lower than desirable.
Kirchoff's Law states that the sum of transmission (Tr), reflectivity (R) and absorptivity (A) for a given wavelength must be equal to one, or Tr+R+A=1.0. For transparent heat-mirrors, solar transmission must be high, and hence the reflectivity and absorptivity must be low. In the infrared, however, the heat-mirror must have high reflectivity and so transmission and absorptivity in the infrared must be low. Using Kirchoff's Law, and assuming that transmission in the infrared is minimal, it can be shown that thermal radiation losses are directly proportional to (1-R), meaning that it is important to have an infrared reflectivity as close to 100% as possible while maintaining the solar transmission as high as possible.
Other heat-mirrors are also known. See Holland, L. and Siddall, G., British Journal of Applied Physics, 9, 359 (1958). These authors tested various metal oxide films on glass, gold films on glass, and gold films sandwiched between either bismuth oxide or silicon monoxide layers. They found that the optimum performance was obtained with multilayer composite having a 130 A gold layer sandwiched between two 450 A bismuth oxide coatings. Nevertheless, the transmittance of this composite was found to be only 73% for green light and the reflectance was only 74% in the near infrared region, values which are not satisfactory for solar energy collection and for many other applications where higher infrared reflectivity coupled with higher transmissions in the visible are required.